Join roof faces together to form a top
In Edit Mode select a vertex, snap a cursor to it (Shift+S-->Cursor to Selected) and change the transformation orientation type to Local. Enable the Snaping Tool (click the 'magnet' icon and change its type to Vertex). Press G and then choose the axis (press X, Y or Z, depending on your situation) twice and move a mouse cursor until it snaps to other vertex. Then select the whole mesh (A) and hit W-->Remove Doubles.